JP 1999-177357 A contains a description concerning a variable gain amplifier using MOS. As shown in FIG. 8 (FIG. 1 in the patent document), it is an example of a variable gain amplifier which can realize a predetermined gain by combining an on-and-off of mutual conductance provided individually with a programmable control.
The Data Sheet AD8332 of Analog Devices, Inc. describes a variable gain amplifier suitable for an ultrasonic diagnostic system. Particularly, as shown in FIG. 9 (FIG. 60 in the document), it is an example of a variable gain amplifier configured with attenuators, in which R-2R ladders and interpolation tap controls are combined together, as well as the subsequent-stage amplifiers, so that it may be provided with logarithmic linear amplifying characteristic in relation to control voltage.
The Data Sheet VCA2616 of Texas Instruments Incorporated has a description concerning a variable gain amplifier suitable for an ultrasonic diagnostic system. In particular, as shown in FIG. 10 (FIG. 12 in the document), it is an example of a variable gain amplifier configured with attenuators, in which resisters and MOSFETs connected in parallel take various on-and-off combinations so as to produce varied voltage division ratio, as well as a subsequent-stage programmable amplifier, so that it may be provided with logarithmic linear amplifying characteristic in relation to control voltage.